Here is the plastic opto interuptor you are after

Thanks for going to the trouble for me - that's exactly what I was after.

I've ordered the motor/gearbox assembly and PCB so once they arrive I'll start with the tinkering.

I'm starting to worry because I'm actually enjoying the challenges of hunting and finding the parts - and I haven't even started to pull anything apart let alone put anything back together. I've even started keeping an eye out for another project to tuck away for later - is this how this obsession starts?